JACKSONVILLE—Although The Who concert here Saturday was billed as one of the biggest summer events, less than 15,000 fans fought the blistering heat in the Gator Bowl to see the British supergroup.

No official count was available but police estimated only between 10,000 and 15,000 mainly college-aged fans turned out for the concert titled "Day on the Green II."

A crowd of 50,000 to 60,000, the approximate number at last year's Rolling Stones show, was anticipated early last week and a spokesman for the promoter was reported as saying 35,000 were needed to meet expenses.

THE HOT, HUMID weather, combined with an early scheduled starting time (2 p.m.) and a $15-at-the-door ticket price, was blamed for the low turnout.

However, The Who, seemingly oblivious to the poor showing, proceeded with nearly two hours of tight, hot rock 'n' roll and masterful stage antics.

The group, living up to a stage madmen reputation they have acquired over the past 10 years, destroyed several pieces of equipment through use — not abuse. They did not set the equipment on fire or intentionally smash equipment as in many of the late 1960s U.S. shows.

With a set consisting primarily of late 1960s hits and work from the "Tommy" album, lead singer Roger Daltry directed the group in visual and vocal harmony. Daltry's and guitarist Pete Townshend's mad-cap stage prancing and leaps combined with the steady beat of Keith Moon's drums kept the audience on its feet throughout the entire performance.

Complimenting, rather than detracting from the set, was a light show in reverse — laser beams coming from the stage into the audience.

THE GREEN AND red streams were aimed at mirrors at the far end of the stadium where they returned to the stage. Thus with the accentuation of the laser effect to Daltry's sound, he mesmerized the fans as the deaf, dumb and blind Tommy he played in the group's movie of the same name.

For those in the audience who were not engulfed in rock when The Who opened shows for Herman's Hermits in 1967, it was the "Tommy" sounds and the group's latest hit, "Squeeze Box," that were most appealing.

But for the older fans, "Magic Bus" and "Substitute" were a return to those show-stealing days — especially with Townshend's, Moon's and bass player John Entwistle's hair now cut in the mid-60's length. Only fair-haired Daltry has retained the flowing locks of the 1970s.

Except for The Who's dazzling performance on a three-story high stage decorated with British flags, the show was much like any other outdoor summer concert. Drugs were prevalent, the heat was stifling and delays in starting and between acts were long.

Fans, wearing cut-offs or bathing suits, doused themselves with water from hoses and showers in an attempt to beat the near 100 degree sun and tried to amuse themselves with massive paper cup fights and Frisbe throwing. Missing though from the concert were the fireworks that have marred recent Tampa events. Only an occasional firecracker explosion was heard.

APPROXIMATELY 200 persons had been treated at the first aid center by 6 p.m., mainly for heat exhaustion. Several fans were arrested for drugs with many more chased through the crowd for various reasons.

"We don't arrest people here unless we have to," one officer said. "We just take them out and talk to them."

Although the show was set to begin at 2 p.m., it was after 3 o'clock when an unannounced Ohio act, Law, began the show with an unidentifiable sound of hard rock combined with Jamaican soul.

Following was Black Oak Arkansas, an obvious favorite of many fans.

Leader Jim Dandy's gutteral voice permeated the audience with raunchy rock sound. His sexual stage antics were the redeeming factor in the performance but were over-shadowed and out-classed once Daltry had begun his dancing act.

FOLLOWING BOA was Labelle, a three-woman black Gospel-soul act. Although the group wore the bright flowing outfits that are part of their act, the late afternoon haze and shadows negated the effect the group tries to achieve in its night performances.

Even their biggest hit "Lady Marmalade," failed to get the crowd excited. But the women did not let the lack of enthusiasm by the audience or a light rain that had begun to fall dampen their attempts to provide an exciting show.